Glycosphingolipids are a class of lipid that having a carbohydrate moiety linked to a ceramide. An exemplary class of glycosphingolipid is the gangliosides. The carbohydrate moiety includes at least one sialic acid moiety. Gangliosides typically include saccharide moieties in addition to the sialic acid moiety and are classified according to the number of monosaccharides and sialic acid groups present in the structure. Gangliosides are known as mono-, di-, tri- or poly-sialogangliosides, depending upon the number of sialic acid residues. Abbreviations employed to identify these molecules include “GM1”, “GD3”, “GT1”, etc., with the “G” standing for ganglioside, “M”, “D” or “T”, etc. referring to the number of sialic acid residues, and the number or number plus letter (e.g., “GT1a”), referring to the elution order in a TLC assay observed for the molecule. See, Lehninger, Biochemistry, p. 294-296 (Worth Publishers, 1981); Wiegandt, Glycolipids: New Comprehensive Biochemistry (Neuberger et al., ed., Elsevier, 1985), pp. 199-260.
For example, the international symbol GM1a designates one of the more common gangliosides, which has been extensively studied. The “M” in the symbol indicates that the ganglioside is a monosialoganglioside and “1” defines its position in a TLC elution profile. The subscripts “a”, “b” or “c” also indicate the positions in a TLC assay of the particular ganglioside. The terminal saccharide is the saccharide, which is located at the end of the carbohydrate moiety, which is opposite to the end that is attached to the ceramide moiety.
The term “glycosphingolipids” (GSLS) refers to a genus that encompasses six classes of compounds, five of which are derived from glucosylceramide (GlcCer), which is enzymatically formed from ceramide and UDP-glucose. The enzyme involved in GlcCer formation is UDP-glucose:N-acylsphingosine glucosyltransferase (GlcCer synthase). The rate of GlcCer formation under physiological conditions may depend on the tissue level of UDP-glucose, which in turn depends on the level of glucose in a particular tissue (Zador, I. Z. et al., J. Clin. Invest. 91: 797-803 (1993)). In vitro assays based on endogenous ceramide yield lower synthetic rates than mixtures containing added ceramide, suggesting that tissue levels of ceramide are also normally rate-limiting (Brenkert, A. et al., Brain Res. 36: 183-193 (1972)).
The level of GSLs controls a variety of cell functions, such as growth, differentiation, adhesion between cells or adhesion between cells and matrix proteins, binding of microorganisms and viruses to cells, and metastasis of tumor cells. In addition, the GlcCer precursor, ceramide, may cause differentiation or inhibition of cell growth (Bielawska, A. et al., FEBS Letters 307: 211-214 (1992)) and be involved in the functioning of vitamin D3, tumor necrosis factor-α, interleukins, and apoptosis (programmed cell death). The sphingols (sphingoid bases), precursors of ceramide, and products of ceramide catabolism, have also been shown to influence many cell systems, possibly by inhibiting protein kinase C(PKC).
A class of glycosphingolipids, the gangliosides are known to be functionally important in the nervous system and it has been demonstrated that gangliosides are useful in the therapy of peripheral nervous system disorders. Numerous gangliosides and derivatives thereof have been used to treat a wide variety of nervous system disorders including Parkinson's disease. Ganglioside GM1, is currently being used in phase II clinical development for the treatment of Parkinson's Disease and cerebral ischemic strokes (see, U.S. Pat. Nos. 4,940,694; 4,937,232; and 4,716,223). Gangliosides have also been used to affect the activity of phagocytes (U.S. Pat. No. 4,831,021) and to treat gastrointestinal disease-producing organisms (U.S. Pat. No. 4,762,822). The gangliosides GM2 and GD2, purified from animal brain, have been conjugated to keyhole limpet hemacyanin (KLH) and mixed with adjuvant QS21, and used to elicit immune responses to these gangliosides, as the basis of a cancer vaccine in phase II and III trials (Progenics, Tarrytown, N.Y.). Ganglioside GM3 is being investigated for use as an anti-cancer agent (WO 98/52577; Nole et al., Exp. Neurology 168: 300-9 (2001)). Glycolipids are also of interest in the treatment of inflammatory bowel disease. See, Tubaro et al., Naunyx-Schmiedebergg's Arch. Pharmacol. 348: 670-678 (1993).
Gangliosides are generally isolated via purification from tissue, particularly from animal brain (GLYCOLIPID METHODOLOGY, Lloyd A. Witting Ed., American Oil Chemists Society, Champaign, III. 187-214 (1976); U.S. Pat. Nos. 5,844,104; 5,532,141; Sonnino et al., J. Lipid Res. 33: 1221-1226 (1992); Sonnino et al., Ind. J. Biochem. Biophys., 25: 144-149 (1988); Svennerhohn, Adv. Exp. Med. Biol. 125: 533-44 (1980)). Gangliosides have also been isolated from bovine buttermilk (Ren et al., J. Bio. Chem. 267: 12632-12638 (1992); Takamizawa et al., J. Bio. Chem. 261: 5625-5630(1986)). Even under optimum conditions, the yields of pure gangliosides, e.g., GM2 and GM3, are vanishingly small. Moreover, purification from mammalian tissue carries with it the risk of transmitting contaminants such as viruses, prion particles, and so forth. Alternate methodologies for securing gangliosides are thus highly desirable.
Despite the many advantages of naturally occurring gangliosides, there is a need for ganglioside analogues that have characteristics, e.g., bioavailability, target specificity, activity, etc. that are enhanced relative to naturally occurring gangliosides. Furthermore, ganglioside analogues, synthetically prepared from sphingosine and sphingosine analogues, are free of the risk of transmission of animal disease, such as bovine spongiform encephalitis.
Due to the importance of gangliosides, efforts have been expended to develop methods of synthesizing pure gangliosides in high yields. Methods of chemically synthesizing gangliosides are described in Hasegawa et al., J. Carbohydrate Chemistry, 11(6): 699-714 (1992) and Sugimoto et al., Carbohydrate Research, 156: C1-C5 (1986). U.S. Pat. No. 4,918,170 discloses the synthesis of GM3 and GM4. Schmidt et al. describe the chemical synthesis of GM3 (U.S. Pat. No. 5,977,329). The references describe multi-step synthetic procedures using laborious protection-activation-coupling-deprotection strategies, at each step of which the intermediate is purified, generally by a combination of extraction and column chromatography. Moreover, none of the synthetic methods is appropriate for the large-scale preparation of gangliosides.
In view of the difficulties associated with the chemical synthesis of carbohydrates, the use of enzymes to synthesize the carbohydrate portions of gangliosides is a promising approach to preparing gangliosides. Enzyme-based syntheses have the advantages of regioselectivity and stereoselectivity. Moreover, enzymatic syntheses can be performed using unprotected substrates. Two principal classes of enzymes are used in the synthesis of carbohydrates, glycosyltransferases (e.g., sialyltransferases, galactosyltransferases), and glycosidases. The glycosidases are further classified as exoglycosidases (e.g., β-galactosidase, β-glucosidase), and endoglycosidases (e.g., endoglycoceramidase). Each of these classes of enzymes has been successfully used to prepare carbohydrates. For a general review, see, Crout et al., Curr. Opin. Chem. Biol. 2: 98-111 (1998) and Arsequell, supra.
Glycosyltransferases have been used to prepare oligosaccharides, and have been shown to be effective for producing specific products with good stereochemical and regiochemical control. For example, β-1,4-galactosyltransferase was used to synthesize lactosamnine, illustrating the utility of glycosyltransferases in the synthesis of carbohydrates (see, e.g., Wong et al., J. Org. Chem. 47: 5416-5418 (1982)). Moreover, numerous synthetic procedures have made use of α-sialyltransferases to transfer sialic acid from cytidine-5′-monophospho-N-acetylneuraminic acid to the 3-OH or 6-OH of galactose (see, e.g., Kevin et al., Chem. Eur. J. 2: 1359-1362 (1996)). For a discussion of recent advances in glycoconjugate synthesis for therapeutic use, see, Koeller et al., Nature Biotechnology 18: 835-841 (2000).
Glycosidases normally catalyze the hydrolysis of a glycosidic bond, however, under appropriate conditions they can be used to form this linkage. Most glycosidases used for carbohydrate synthesis are exoglycosidases; the glycosyl transfer occurs at the non-reducing terminus of the substrate. The glycosidase takes up a glycosyl donor in a glycosyl-enzyme intermediate that is either intercepted by water to give the hydrolysis product, or by an acceptor, to give a new glycoside or oligosaccharide.
In addition to the need for an array of synthetic ganglioside analogues having improved therapeutic properties, there remains a need for a simple, high-yielding procedure to prepare the synthetic ganglioside analogues. Since the biological activity of a ganglioside or a synthetic analogue thereof generally depends upon the presence of a particular glycoform, or the absence of a particular glycoform, a need exists for an in vitro procedure to enzymatically prepare a ganglioside analogue with a pre-selected glycosylation pattern, particularly on substrates such as ceramide, sphingosine and their analogues. The present invention is directed to addressing these, and other needs.